pgsql: Doc: Fix example related to partition pruning

Doc: Fix example related to partition pruning

Append node has been removed in v12 when there would be only one subnode
under it.

Author: Amit Langote
Discussion: https://postgr.es/m/CA+HiwqHhS62w8zUFXF4NBjvMboCXYnD-jWoWp-tfo2aHvP3Gxg@mail.gmail.com
Backpatch-through: 12

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f5daf7f3266ff5a92f1bf8d386bd5ac3d7d042d6

Modified Files
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doc/src/sgml/ddl.sgml | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)